As internal combustion engines for vehicles, there are known bi-fuel engines including a liquid fuel injection unit to inject liquid fuel, such as gasoline and a gaseous fuel injection unit to inject gaseous fuel, such as natural gas (CNG: compressed natural gas). It is known in the art for a bi-fuel engine to mount a liquid injection unit above intake runners in a manner that it is directed toward the upper surface of each of the intake runners and a gaseous fuel injection unit within a narrow space between the intake runners and a cylinder head (see patent literature 1 for example). In the bi-fuel engine, it is in such narrow space between the intake runners and the cylinder head to connect the gaseous fuel injection unit to the intake runners at lower portions (i.e. portions of the intake runners facing downwards).